An organic EL display device is well-known as a thin-screen, high-definition display device with low power consumption. An organic EL display device contains a plurality of pixel circuits arranged in a matrix, each pixel circuit including an organic EL element formed by a light-emitting electro-optic element that is driven by an electric current, a driving transistor and the like.
Methods for controlling an amount of current flowing in current-driven display devices such as organic EL elements can be broadly classified as being either constant current control methods (or current program-type driving methods) whereby the current to flow in the display device is controlled using a data signal current flowing in the data signal line electrode of the display device, or constant voltage control methods (or voltage program-type driving methods) whereby the current to flow in the display device is controlled using a voltage dependent on a data signal voltage. When display is performed with an organic EL display device using the constant voltage control method, it is necessary to compensate for variations in threshold voltage and mobility of the driving transistors, which are typically thin-film transistors (hereinafter abbreviated to “TFTs”), and for current reductions (loss of brightness) that occur as the resistance of the organic EL elements increases due to degradation over time. On the other hand, when the constant current control method is used, it not generally necessary to perform the above-described compensation because the data signal current value is controlled so that a fixed current flows in the organic EL element irrespective of the above-described threshold voltage or internal resistance of the organic EL element. However, it is common knowledge that when the constant current control method is used, the number of driving transistors and amount of wiring are higher than when the constant voltage control method used, thus reducing the aperture ratio. Also, since the data signal current is weak, it is not possible to rapidly write data using the charge on the data signal line electrodes or the like.
In configurations employing the constant voltage control method, there are various conventional configurations for the pixel circuit that performs the above-described compensation. For example, Japanese Patent Application Laid-Open Publication No. 2005-31630 discloses an organic EL display device in which compensation for variation in the threshold voltage is performed by providing a transistor for detecting fluctuation of the threshold voltage of the driving transistors in the pixel circuit. Note that in the following, compensation for variation in the threshold voltage is also referred to as “threshold voltage compensation”. Further, Japanese Patent Application Laid-Open Publication No. 2007-233326 discloses an organic EL display device in which compensation for variation in the transistor characteristics, and variation (deviation) in mobility in particular, is performed by detecting the driving current flowing in the driving transistor and controlling the voltage supplied to the data line in accordance with the detection results.